In many security applications, fixed cameras, pan-tilt-zoom (PTZ) cameras, and short range surface detection radars are mounted on towers to provide complete coverage of the secured area. These towers are prone to slowly varying deformations caused by temperature gradients and wind loading which causes each sensor bore sight to rotate away from its registered value. To correct for these errors it is possible to mount compass and inclinometer boards on each sensor, but in large installations there would be many such boards to calibrate and maintain. A need therefore exists for improved methods and systems for determining and removing the effects of the bias in the sensors.